Been struggling with a bad hesitation/stumble problem.

After changing intakes gaskets (may just be a coincidence) starting having a bad hestition stumble with car when WOT or giving some extra gas when accelerating.

Took to local Ford guru - he called me told me he got rid of the stumble. Went and picked the car up and not a mile down the road starting having the same problem. Took it back - he swore it was running fine for him. We played with timing and springs on the the vacuum advance took for short test drives - about a block seem to run good not a speed demon but pretty good. Now one the way home same thing - 1/2 mile out same thing - hesitation & stumble with backfiring thru carb is back. Whatever the problem its seems okay on these short spurts as we were tweaking but problem comes back when it's driven more than block test drives we have been doing. I'll check timing tomorrow to seem if it's still where we set it tonght but really could use some ideas on what may be causing it.

Some background on the car:
1. New Holley, fuel lines, sending unit, choke is fine - don't beelive it is fuel related
2. Pertronix ignition
3. Good vacuum 20 psi
4. Spark is good tested that
5. Just put new vacuum advance thought that may be the problem

302 motor, 1970 stang.

Appreciate any help - starting to run out of ideas.

Thanks

 

Well im gonna say fuel pump, when was it last replaced? Sounds like its not puting out the volume anymore,

Gun

 

When I changed the lines about a month ago - also changed the pump.

 

I'd suggest you get it on a scope, and do what it takes to repeat the problem. 9 times out of 10, popping through the carb and intermittent hesitation/stumbling is an ignition issue.
A good scope read will tell you right were to go.

Good luck,

Scott

 

 

I was thinking it was ignition also. Only thing throwing me is that problem really is not intermittent. Basically always occurs when car has been run under load for a few minutes. When first taking off - no hestitation/stumbling- runs good - comes as car is run for awhile couple minutes. That is why the first couple of times we test drove thought it was fine - drove up and down the block. The only 2 things I did different when changing intake gasket was plug exhaust ports on manifold under carb and re ran wire to flame thrower to make sure it was getting 12 volts. Doubt if it is related.

 

Well, maybe you could try unplugging the heat riser port and see if it goes away. Cuz to me it sounds like theres a heat issue, as in the intake charge is too hot, or your exhaust is not getting scavenged all the way and building up. Just curious why would you plug up the heat riser ports anyway? There is hardly any performance gain from plugging them(maybe 1 hp). That and make sure the springs you got for the disty are the right ones.
-P.

 

you checking your timing with the vaccum advance hooked up?

 

How are your sparkplug wires bundled? I had this problem, my wires were bundled together and they were crossfireing. After separating them every thing worked fine

 

For the exhaust crossover on manifold I was told by Edelbrock tech that it would run cooler and if it was a fair weather car you should go ahead and use the plugs. They said prirmary purpose of cross over was to warm up intake by having heat pass thru for to help choke function properly in cold weather. I was thinking I would pull manifold back off and unplug and see if goes away - was saving that for last resort since I have been told by everyone that I inquired that this would not cause the problem.